U.S. patent number 7,374,537 [Application Number 10/951,853] was granted by the patent office on 2008-05-20 for performing ultrasound ranging in the presence of ultrasound interference.
This patent grant is currently assigned to Boston Scientific Scimed, Inc.. Invention is credited to N. Parker Willis.
United States Patent |
7,374,537 |
Willis |
May 20, 2008 |
Performing ultrasound ranging in the presence of ultrasound
interference
Abstract
A distance measuring system comprises first and second
transducers, and an ultrasound ranging subsystem coupled to the
first and second transducers for performing a plurality of distance
measurements between the first and second transducers. The distance
measurement system can have various applications, including medical
applications, in which case, the first and second transducers can
be mounted on a catheter. The distance measuring system further
comprises a filter coupled to the ultrasound ranging subsystem for
filtering ultrasound interference from the plurality of distance
measurements (such as, e.g., eight), and outputting a distance
based on the filtered distance measurements. The filter filters the
ultrasound interference by selecting one of the plurality distance
measurements, in which case, the outputted distance is the selected
distance measurement. Because the ultrasound interference will
typically represent itself as the shortest distance measurement,
the selected distance measurement is preferably greater than the
minimum distance measurement (such as, e.g., the maximum distance
measurement), thereby filtering the ultrasound interference
out.
Inventors: |
Willis; N. Parker (Atherton,
CA) |
Assignee: |
Boston Scientific Scimed, Inc.
(Maple Grove, MN)
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Family
ID: |
31494650 |
Appl.
No.: |
10/951,853 |
Filed: |
September 27, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050038341 A1 |
Feb 17, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10214441 |
Aug 6, 2002 |
6805132 |
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Current U.S.
Class: |
600/438;
367/99 |
Current CPC
Class: |
A61B
5/064 (20130101); A61B 5/065 (20130101); A61B
8/0841 (20130101); G01S 11/14 (20130101); A61B
8/0833 (20130101); G01S 5/30 (20130101) |
Current International
Class: |
A61B
8/00 (20060101); G01S 15/00 (20060101) |
Field of
Search: |
;600/437-438,443-447,454-456,459-471 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jaworski; Francis J.
Attorney, Agent or Firm: Vista IP Law Group LLP
Parent Case Text
RELATED APPLICATION DATA
This application is a continuation of U.S. patent application Ser.
No. 10/214,441, filed Aug. 6, 2002, now U.S. Pat. No. 6,805,132.
Claims
What is claimed is:
1. A distance measuring system, comprising: first and second
transducers; an ultrasound ranging subsystem coupled to the first
and second transducers for performing straight path distance
measurements between the first and second transducers; and a filter
coupled to the ultrasound ranging subsystem for receiving the
distance measurements, comparing the distance measurements to each
other, determining a relative ordering of the distance measurements
based on the comparison, selecting one of the distance measurements
based on the determined distance measurement ordering, and
outputting the selected distance measurement as the distance
between the first and second transducers.
2. The system of claim 1, wherein the distance measurement is only
selected if it is greater than the minimum distance
measurement.
3. The system of claim 1, wherein the distance measurement is only
selected if it is the maximum distance measurement.
4. The system of claim 1, wherein the number of distance
measurements compared is greater than 2.
5. The system of claim 1, wherein the ultrasound ranging subsystem
further comprises: a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer; a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector;
and measurement means coupled to the pulse generator and the
threshold detector; wherein, for each distance measurement, the
measurement means triggers the pulse generator to generate and
transmit a transmit pulse to the first transducer, measures the
elapsed time between transmission of the transmit pulse and
detection of a receive pulse by the threshold detector, and
generates the distance measurement based on the measured elapsed
time.
6. The system of claim 5, wherein the measurement means comprises a
digital counter for measuring the elapsed time.
7. The system of claim 1, further comprising a catheter on which at
least one of the first and second transducers is mounted.
8. The system of claim 1, further comprising a processor coupled to
the filter for determining the position of one of the first and
second transducers in a three-dimensional coordinate system at
least partially based on the outputted distance measurement.
9. A distance measuring system, comprising: first and second
transducers; an ultrasound ranging subsystem coupled to the first
and second transducers for performing straight path distance
measurements between the first and second transducers; and a filter
coupled to the ultrasound ranging subsystem for sequentially
receiving the distance measurements, filtering ultrasound
interference from the last N distance measurements, and outputting
one of the N distance measurements as the distance between the
first and second transducers based on the filtering of the distance
measurements, wherein N is greater than 2.
10. The system of claim 9, wherein the ultrasound interference
filtering comprises comparing the N distance measurements, and
wherein the distance measurement is output based on the distance
measurement comparison.
11. The system of claim 10, wherein the ultrasound interference
filtering comprises determining a distance measurement that is
greater than a minimum of the N distance measurements, and wherein
the determined distance measurement is output as the distance
between the first and second transducers.
12. The system of claim 9, wherein the ultrasound interference
filtering comprises determining a maximum of the N distance
measurements, and wherein the determined distance measurement is
output as the distance between the first and second
transducers.
13. The system of claim 9, wherein N is at least 8.
14. The system of claim 9, wherein the ultrasound ranging subsystem
further comprises: a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer; a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector;
and measurement means coupled to the pulse generator and the
threshold detector; wherein, for each distance measurement, the
measurement means triggers the pulse generator to generate and
transmit a transmit pulse to the first transducer, measures the
elapsed time between transmission of the transmit pulse and
detection of a receive pulse by the threshold detector, and
generates the distance measurement based on the measured elapsed
time.
15. The system of claim 14, wherein the measurement means comprises
a digital counter for measuring the elapsed time.
16. The system of claim 9, further comprising a catheter on which
at least one of the first and second transducers is mounted.
17. The system of claim 9, further comprising a processor coupled
to the filter for determining the position of one of the first and
second transducers in a three-dimensional coordinate system at
least partially based on the outputted distance measurement.
18. A distance measuring system, comprising: first and second
transducers; an ultrasound ranging subsystem coupled to the first
and second transducers for performing distance measurements between
the first and second transducers; and a filter coupled to the
ultrasound ranging subsystem for sequentially receiving the
distance measurements, comparing the last N distance measurements
to each other, determining a relative ordering of the last N
distance measurements based on the comparison, detecting and
comparing ultrasound interference against a threshold, selecting
one of the last N distance measurements based on the determined
distance measurement ordering if the detected ultrasound
interference is above the threshold, selecting the Nth distance
measurement if the detected ultrasound is below the threshold, and
outputting the selected distance measurement as the distance
between the first and second transducers.
19. The system of claim 18, wherein the selected one of the last N
distance measurements must be greater than a minimum of the last N
distance measurements.
20. The system of claim 18, wherein the selected one of the last N
distance measurements must be the maximum of the last N distance
measurements.
21. The system of claim 18, wherein N is greater than 2.
22. The system of claim 18, wherein N is at least 8.
23. The system of claim 18, wherein the ultrasound ranging
subsystem further comprises: a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer; a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector;
and measurement means coupled to the pulse generator and the
threshold detector; wherein, for each distance measurement, the
measurement means triggers the pulse generator to generate and
transmit a transmit pulse to the first transducer, measures the
elapsed time between transmission of the transmit pulse and
detection of a receive pulse by the threshold detector, and
generates the distance measurement based on the measured elapsed
time.
24. The system of claim 23, wherein the measurement means comprises
a digital counter for measuring the elapsed time.
25. The system of claim 18, further comprising a catheter on which
at least one of the first and second transducers is mounted.
26. The system of claim 18, further comprising a processor coupled
to the filter for determining the position of one of the first and
second transducers in a three-dimensional coordinate system at
least partially based on the outputted distance measurement.
27. The system of claim 18, wherein the ultrasound interference
detection and comparison comprises computing a distance variation
of the last N distance measurements and comparing the distance
variation to a threshold value.
28. The system of claim 27, wherein the distance variation is the
difference between the maximum and minimum of the last N distance
measurements.
29. The system of claim 27, wherein the distance variation is the
variance of the last N distance measurements.
30. The system of claim 27, wherein the distance variation is the
second derivative of the last N distance measurements.
31. An ultrasound system, comprising: a first ultrasound-based
subsystem for performing a distance measuring function; and a
second ultrasound-based subsystem for generating ultrasound energy
used to perform a function different from the distance measuring
function; wherein the first ultrasound-based subsystem includes:
first and second transducers; an ultrasound ranging subsystem
coupled to the first and second transducers for performing distance
measurements between the first and second transducers; and a filter
coupled to the ultrasound ranging subsystem for receiving the
distance measurements, and filtering the ultrasound energy from the
distance measuring function by comparing the distance measurements
to each other, and selecting one of the distance measurements based
on the comparison.
32. The system of claim 31, wherein the filter outputs the selected
distance measurement as the distance between the first and second
transducers.
33. The system of claim 32, wherein the distance measurement is
only selected if it is greater than the minimum distance
measurement.
34. The system of claim 32, wherein the distance measurement is
only selected if it is the maximum distance measurement.
35. The system of claim 31, wherein the number of distance
measurements is greater than 2.
36. The system of claim 35, wherein the measurement means comprises
a digital counter for measuring the elapsed time.
37. The system of claim 31, wherein the ultrasound ranging
subsystem further comprises: a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer; a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector;
and measurement means coupled to the pulse generator and the
threshold detector; wherein, for each distance measurement, the
measurement means triggers the pulse generator to generate and
transmit a transmit pulse to the first transducer, measures the
elapsed time between transmission of the transmit pulse and
detection of a receive pulse by the threshold detector, and
generates the distance measurement based on the measured elapsed
time.
38. The system of claim 31, further comprising a catheter on which
at least one of the first and second transducers is mounted.
39. The system of claim 31, further comprising a processor coupled
to the filter for determining the position of one of the first and
second transducers in a three-dimensional coordinate system at
least partially based on the outputted distance measurement.
40. The system of claim 31, wherein second ultrasound-based
subsystem is an ultrasound imaging subsystem.
41. The system of claim 40, wherein the ultrasound energy filtering
comprises comparing the N distance measurements to each other, and
wherein the distance measurement is output based on the distance
measurement comparison.
42. The system of claim 40, wherein N is at least 2.
43. The system of claim 42, wherein the measurement means comprises
a digital counter for measuring the elapsed time.
44. The system of claim 40, further comprising a catheter on which
at least one of the first and second transducers is mounted.
45. The system of claim 40, further comprising a processor coupled
to the filter for determining the position of one of the first and
second transducers in a three-dimensional coordinate system at
least partially based on the outputted distance measurement.
46. The system of claim 40, wherein second ultrasound-based
subsystem is an ultrasound imaging subsystem.
47. The system of claim 40, wherein the ultrasound ranging
subsystem further comprises: a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer; a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector;
and measurement means coupled to the pulse generator and the
threshold detector; wherein, for each distance measurement, the
measurement means triggers the pulse generator to generate and
transmit a transmit pulse to the first transducer, measures the
elapsed time between transmission of the transmit pulse and
detection of a receive pulse by the threshold detector, and
generates the distance measurement based on the measured elapsed
time.
48. An ultrasound system, comprising: a first ultrasound-based
subsystem for performing a distance measuring function; and a
second ultrasound-based subsystem for generating ultrasound energy
used to perform a function different from the distance measuring
function; wherein the first ultrasound-based subsystem includes:
first and second transducers; an ultrasound ranging subsystem
coupled to the first and second transducers for performing a
plurality of distance measurements between the first and second
transducers; and a filter coupled to the ultrasound ranging
subsystem for sequentially receiving the distance measurements,
filtering the ultrasound energy from the last N distance
measurements and outputting one of the N distance measurements as
the distance between the first and second transducers based on the
filtering of the distance measurements.
49. The system of claim 48, wherein the ultrasound energy filtering
comprises determining a distance measurement that is greater than a
minimum of the N distance measurements, and wherein the determined
distance measurement is output as the distance between the first
and second transducers.
50. The system of claim 48, wherein the ultrasound energy filtering
comprises determining a maximum of the N distance measurements, and
wherein the determined distance measurement is output as the
distance between the first and second transducers.
Description
FIELD OF THE INVENTION
The invention relates generally to ultrasound ranging, and more
particularly to systems and methods for performing ultrasound
ranging in the presence of ultrasound interference.
BACKGROUND OF THE INVENTION
Ultrasound ranging is a technique for computing the distance
between two ultrasound transducers. The principle of ultrasound
ranging is illustrated in FIG. 1, which shows two ultrasound
transducers 10,20 separated by a distance. One of the ultrasound
transducers is designated as a transmitting transducer 10 and the
other is designated as a receiving transducer 20. To measure the
distance between the transducers 10,20, the transmitting transducer
10 transmits an ultrasound pulse 25, which is detected by the
receiving transducer 20. The distance, d, between the transducers
10,20 is computed as d=v.tau. where v is the velocity of the
ultrasound pulse 25 in the medium between the transducers 10,20 and
.tau. is the time of flight of the ultrasound pulse 25 in traveling
from the transmitting transducer 10 to the receiving transducer
20.
One application of ultrasound ranging is in ultrasound positional
tracking to track the position of a device within a
three-dimensional (3-D) coordinate system. Referring to FIG. 2,
this is accomplished by mounting one or more ranging transducers
110 on the device 115 being tracked and providing four or more
reference transducers 120-1 to 1204 that are spaced apart. In this
particular example, the device 115 being tracked is a catheter tip.
The ranging transducer 110 acts as a receiving transducer and each
of the reference transducers 120-1 to 120-4 can act both as a
receiving and transmitting transducer.
To establish the 3-D coordinate system, the reference transducers
120-1 to 120-4 are sequentially excited to transmit ultrasound
pulses (not shown). When each reference transducer 120-1 to 120-4
transmits an ultrasound pulse, the other reference transducers
120-1 to 120-4 detect the ultrasound pulse. The relative distances
between the reference transducers 120-1 to 120-4 are then computed
by performing ultrasound ranging on each of the detected ultrasound
pulses. The computed distances are then triangulated to determine
the relative positions between the reference transducers 120-1 to
120-4 in 3-D space. The relative positions between the reference
transducers 120-1 to 120-4 are then mapped onto the 3-D coordinate
system to provide a reference for tracking the position of the
ranging transducer 110 in the 3-D coordinate system.
To track the position of the ranging transducer 110, and hence the
device 115 carrying the ranging transducer 110, in the 3-D
coordinate system, the reference transducers 120-1 to 120-4 are
sequentially excited to transmit ultrasound pulses. When each of
the reference transducers 120-1 to 120-4 transmits an ultrasound
pulse, the ranging transducer 110 detects the ultrasound pulse. The
distance d1-d4 between the ranging transducer 110 and each of the
reference transducers 120-1 to 120-4 is computed by performing
ultrasound ranging on each of the detected ultrasound pulses. The
computed distances are then triangulated to determine the relative
position of the ranging transducer 110 to the reference transducers
120-1 to 120-4 in 3-D space. The position of the ranging transducer
110 in the 3-D coordinate system is then determined based on the
relative position of the ranging transducer 110 to the reference
transducers 120-1 to 120-4 and the known positions of reference
transducers 120-1 to 1204 in the 3-D coordinate system.
An example of a tracking system using ultrasound ranging is the
Realtime Position Management.TM. (RPM) tracking system developed
commercially by Cardiac Pathways Corporation, now part of Boston
Scientific Corp. The RPM system uses ultrasound ranging to track
the positions of medical devices, including reference catheters,
mapping catheters and ablation catheters.
Because ultrasound ranging relies on the transmission and detection
of ultrasound pulses to measure distance, it is vulnerable to
ultrasound interference from ultrasound sources, e.g., an
ultrasound imager. For example, ultrasound interference may be
detected by the receiving transducer 20 and misinterpreted as an
ultrasound pulse from the transmitting transducer 10, producing an
erroneous distance measurement.
Therefore, there is need for systems and methods that enable the
use of ultrasound ranging in the presence of ultrasound
interference.
SUMMARY OF THE INVENTION
The present inventions are directed to systems and methods that
enable the use of ultrasound measuring equipment in the presence of
ultrasound interference.
In accordance with a first aspect of the present inventions, a
distance measuring system comprises first and second transducers,
and an ultrasound ranging subsystem coupled to the first and second
transducers for performing a plurality of distance measurements
between the first and second transducers. By way of non-limiting
example, the distance measuring system, in performing the distance
measurements, comprises a pulse generator coupled to the first
transducer for generating and transmitting transmit pulses to the
first transducer, a threshold detector coupled to the second
transducer for detecting receive pulses from the second detector,
and measurement means (e.g., a digital counter) coupled to the
pulse generator and the threshold detector. In this case, for each
distance measurement, the measurement means triggers the pulse
generator to generate and transmit a transmit pulse to the first
transducer, measures the elapsed time between transmission of the
transmit pulse and detection of a receive pulse by the threshold
detector, and generates the distance measurement based on the
measured elapsed time.
The distance measuring system further comprises a filter coupled to
the ultrasound ranging subsystem for filtering ultrasound
interference from the plurality of distance measurements (such as,
e.g., eight), and outputting a distance based on the filtered
distance measurements. The distance measurement system can have
various applications, including medical applications, in which
case, the first and second transducers can be mounted on a
catheter.
In the preferred embodiment, the filter filters the ultrasound
interference by selecting one of the plurality distance
measurements, in which case, the outputted distance is the selected
distance measurement. Because the ultrasound interference will
typically represent itself as the shortest distance measurement,
the selected distance measurement is preferably greater than the
minimum distance measurement (such as, e.g., the maximum distance
measurement), thereby filtering the ultrasound interference
out.
Although the present inventions should not be so limited in its
broadest aspects, the filter sequentially receives the distance
measurements, and filters the ultrasound interference from the last
N distance measurements. In this case, the filter may filter the
last N distance measurements by selecting one of them. So that the
system is more responsive to movements of the transducers, the
filter can compute a distance variation of the N distance
measurements, and compare the distance variation to a threshold
value. The filter can then output the distance when the distance
variation exceeds the threshold value, while outputting the most
recent distance measurement received from the ultrasound ranging
subsystem otherwise. In effect, the filtering is only accomplished
when there is ultrasound interference, thereby providing more
responsiveness to the distance measuring process. The distance
variation computation can be accomplished in a variety of ways,
including taking the difference between the maximum and minimum of
the last N distance measurements, calculating the variance of the
last N distance measurements, or calculating the second derivative
of the last N distance measurements.
In accordance with a second aspect of the present inventions, a
method for measuring the distance between two transducers comprises
performing a plurality of distance measurements (e.g., eight)
between the transducers. For example, the distance measurement can
comprise exciting one of the transducers to transmit an ultrasound
pulse, and measuring the time for the ultrasound pulse to reach the
other transducer.
The method further comprises filtering ultrasound interference from
the plurality of distance measurements, and outputting a distance
based on the filtered distance measurements. The distance
measurements can be filtered by, e.g., selecting one of the
plurality distance measurements, in which case, the outputted
distance will be the selected distance measurement. The selected
distance measurement is preferably more than the minimum distance
measurement, such as, e.g., the maximum distance measurement.
Although the present inventions should not be some limited in its
broadest aspects, the distance measurements are sequentially
received, and the ultrasound interference is filtered from the last
N distance measurements. In this case, the last N distance
measurements can be filtered by selecting one of them. So that the
method is more responsive to movements of the transducers, a
distance variation of the N distance measurements can be computed
and compared to a threshold value. The distance can then be
outputted when the distance variation exceeds the threshold value,
while the most recent distance measurement received from the
ultrasound ranging subsystem can be outputted otherwise, thereby
providing for a more responsive method. The distance variation
computation can be accomplished in a variety of ways, including
taking the difference between the maximum and minimum of the last N
distance measurements, calculating the variance of the last N
distance measurements, or calculating the second derivative of the
last N distance measurements.
BRIEF DESCRIPTION OF THE DRAWINGS
The drawings illustrate the design and utility of preferred
embodiments of the present invention, in which similar elements are
referred to by common reference numerals. In order to better
appreciate how the above-recited and other advantages and objects
of the present inventions are obtained, a more particular
description of the present inventions briefly described above will
be rendered by reference to specific embodiments thereof, which are
illustrated in the accompanying drawings. Understanding that these
drawings depict only typical embodiments of the invention and are
not therefore to be considered limiting of its scope, the invention
will be described and explained with additional specificity and
detail through the use of the accompanying drawings in which:
FIG. 1 is a diagram illustrating the principle of ultrasound
ranging between two transducers.
FIG. 2 is a diagram illustrating ultrasound positional tracking
using ultrasound ranging.
FIG. 3 is a functional diagram of an ultrasound ranging system.
FIG. 4 is a timeline of a transmit pulse and a received pulse used
to measure the time of flight of an ultrasound pulse.
FIG. 5 is a diagram of the ultrasound ranging system of FIG. 3 in
an environment containing a source of ultrasound interference.
FIG. 6 is a timeline depicting the arrival of ultrasound
interference between the transmit pulse and the receive pulse.
FIG. 7 is a functional diagram of the ultrasound ranging system
further comprising a distance filter according to one embodiment of
the invention.
FIG. 8 is a flowchart illustrating the operation of a distance
filter according to another embodiment of the invention.
FIG. 9 is a functional block diagram of an ultrasound ranging
system comprising multiple transmitting transducers according to
another embodiment of the invention.
FIG. 10 is a functional block diagram of an ultrasound positional
tracking system according to still another embodiment of the
invention.
FIG. 11 illustrates examples of a medical catheter and a reference
catheter that can be used with the system of FIG. 10.
FIG. 12 illustrates an exemplary display image of the system of
FIG. 10.
FIG. 13 is a graph illustrating the probability of a distance error
with and without the filter of the invention as a function of
distance between two transducers.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 is a block diagram of an ultrasound ranging system 310 for
measuring the distance between transducers 10,20. The ranging
system 310 generally includes a pulse generator 320 coupled to the
transmitting transducer 10, a threshold detector coupled 330 to the
receiving transducer 20, and distance circuitry 340 coupled to the
threshold detector 330. The pulse generator 320 may generate
voltage pulses having a frequency of, e.g., 600 KHz. The threshold
detector 330 detects signals from the receiving transducer 20 that
are above a threshold level, e.g., a voltage level. The ranging
system 310 further includes control and timing circuitry 350
coupled to the pulse generator 320, and a distance counter 360
coupled to the control and timing circuitry 350 and the distance
circuitry 340. The distance counter 360 may be a digital counter
driven by a clock. For ease of discussion, only one transmitting
transducer 10 is shown. The more typical case of multiple
transmitting transducers will be discussed later.
To measure the distance between the transducers 10,20, the control
and timing circuitry 350 triggers the pulse generator 320 to
generate and transmit a transmit pulse to the transmitting
transducer 10. The transmitting transducer 10 converts the transmit
pulse into an ultrasound pulse and transmits the ultrasound pulse
25. The control and timing circuitry 350 also triggers the distance
counter 360 to begin counting from zero at the transmit time of the
transmit pulse. The running count value of the distance counter 360
provides a measure of time from the transmission of the transmit
pulse.
After the ultrasound pulse 25 has been transmitted, the receiving
transducer 20 receives the ultrasound pulse and converts the
ultrasound pulse into a receive pulse, which is detected by the
threshold detector 330. Upon detection of the receive pulse, the
distance circuitry 340 reads the current count value from the
distance counter 360. The read count value indicates the elapsed
time between the transmission of the transmit pulse and the
detection of the receive pulse, which corresponds to the time of
flight, .tau., of the ultrasound pulse 25 between the transducers
10,20. This is illustrated in FIG. 4, which shows a timeline of the
transmit pulse and the receive pulse. The read count value also
provides a distance measurement between the transducers 10,20. This
is because the distance, d, between the transducers 10,20 is
proportional to the time of flight, .tau., of the ultrasound pulse
25, by d=v.tau. where v is the velocity of the ultrasound pulse 25.
The distance circuitry 340 outputs the read count value as a
distance measurement between the transducers 10,20.
In one embodiment, the distance circuitry 340 listens for the
receive pulse within a time window, e.g., 100 .mu.sec, after the
transmit pulse has been transmitted. The time window may begin
immediately or shortly after the transmit pulse has been
transmitted. In determining the time of detection of the receive
pulse, the distance circuitry 340 interprets the first signal that
the threshold detector 330 detects within the time window as the
receive pulse.
The operation of the ultrasound ranging system 310 in an
environment containing ultrasound interference will now be
described with reference to FIG. 5, in which a source 510 of
ultrasound interference 525 is introduced. The source of ultrasound
interference may be, e.g., an ultrasound imaging transducer. In the
following discussion, it will be assumed that the ultrasound
interference 525 is large enough in amplitude to be detected by the
threshold detector 330.
When the ultrasound interference reaches the receiving transducer
20 before transmission of the transmit pulse, the distance
circuitry 340 ignores the ultrasound interference. This is because
the distance circuitry 340 does not listen for the receive pulse
until after the transmit pulse has been transmitted.
FIG. 6 is a timeline illustrating the case in which the ultrasound
interference reaches the receiving transducer 20 between the
transmit pulse and the receive pulse. When this occurs, the
threshold detector 330 detects the ultrasound interference first.
As a result, the distance circuitry 340 misinterprets the detected
ultrasound interference as the receive pulse. This causes the
distance measurement 350 unit to prematurely read the count value
from the distance counter 360. In this case, the read count value
indicates the time between the transmit pulse and the detection of
the ultrasound interference. As a result, the read count value
corresponds to an erroneous time of flight, .tau..sub.err, that is
shorter than the actually time of flight, .tau., of the ultrasound
pulse, as illustrated in FIG. 6. This in turn causes the distance
circuitry 340 to output a distance measurement that is shorter than
the actual distance between the transducers 10,20.
When the ultrasound interference reaches the receiving transducer
20 after the receive pulse, the distance measurement circuitry 340
ignores the ultrasound interference. This is because the threshold
detector 330 detects the receive pulse first. As a result, the
distance circuitry 340 correctly reads the count value at the
detection of the receive pulse.
Of the three cases discussed above, the only case in which the
ranging system 310 is affected by the ultrasound interference is
when the ultrasound interference reaches the receiving transducer
20 between the transmit pulse and the receive pulse. In this case,
the distance circuitry 340 outputs a distance measurement that is
shorter than the actual distance between the transducers 10,20.
Thus, ultrasound interference causes the ranging system 310 to
measure distances that are too short. The invention exploits this
property to filter out erroneous distance measurements caused by
ultrasound interference, as explained further below.
FIG. 7 illustrates an embodiment of a system 710 for measuring the
distance between transducers 10,20 further including a distance
filter 715 coupled to the distance circuitry 340. In the preferred
embodiment, the distance filter 715 is implemented in software, but
can be implemented in firmware or hardware as well. In this
embodiment, the control and timing circuitry 350 may continuously
initiate distance measurements between the transducers 10,20 at
regular intervals (e.g., once every 13 ms for a RPM system). Each
time the control and timing circuitry 340 initiates a distance
measurement, the distance circuitry 340 outputs a distance
measurement to the distance filter 715. The filter 715 takes the
last N distance measurements outputted by the distance circuitry
340, and outputs the maximum of the N distance measurements, where
N is a positive integer (e.g., N=8).
The operation of the distance filter 715 can be represented as
y(n)=max[x(n), x(n-1), . . . , x(n-N)] where y(n) is the most
recent output of the distance filter, x(n) is the most recent
distance measurement from the distance circuitry 340, and max[ ] is
a function that takes the maximum of the last N distance
measurements from the distance circuitry 340 starting with the most
recent distance measurement x(n).
The distance filter 715 filters out erroneous distance measurements
caused by ultrasound interference. This is because ultrasound
interference causes the distance circuitry 340 to output distance
measurements that are shorter than the actual distance between the
transducers 10,20. As a result, correct distance measurements
outputted by the distance circuitry 340 will be larger than
erroneous distance measurements caused by ultrasound interference.
Therefore, when at least one of the last N distance measurements is
correct, the maximum distance measurement outputted by the distance
filter 715 will be one of the correct distance measurements. The
distance filter 715 only outputs a distance error when every one of
the last N distance measurements from the distance circuitry 340 is
in error.
The distance filter 715 of the invention can significantly reduce
distance errors due to ultrasound interference. This can be
demonstrated by assuming that the probability of a distance error
from the distance circuitry 340 due to ultrasound interference is
P. In this case, the probability that every one of the last N
distance measurements is in error is P to the Nth power. Since the
distance filter 715 outputs a distance error only when every one of
the last N distance measurements is in error, the probability of a
distance error from the distance filter 715 is P to the Nth power,
which can be significantly smaller than P. For example, if P equals
77% and N=8, the probability of a distance error from the distance
circuitry 340 due to ultrasound interference is 77%, while the
probability of a distance error from the distance filter due to
ultrasound interference is 12.3%. Obviously increasing N can
further reduce the probability of a distance error from the
distance filter 715 due to ultrasound interference. However,
increasing N may increase another type of distance error, as
explained further below.
For the case in which the control and timing circuitry 350
initiates distance measurements at regular intervals (e.g., once
every 13 milliseconds), the distance filter 715 outputs the maximum
distance measurement over a finite measurement time window, M, of
M=N.DELTA.t where .DELTA.t is the time between adjacent distance
measurements and N is the number of distance measurements
considered. For example, when .DELTA.t=13 ms and N=8, the
measurement time window is 104 ms. In order for the distance filter
715 to provide a good approximation of the current distance between
the transducers 10,20, the distance between the transducers 10,20
should remain relatively stable within the measurement window. This
ensures that the maximum distance measurement within the
measurement window provides a good approximation of the current
distance between the transducers 10,20. When the distance between
the transducers 10,20 varies within measurement window, the maximum
distance may no longer closely approximate the current distance
between the transducers 10,20. For example, when the distance
between the transducers 10,20 decreases within the measurement
window, the maximum distance will tend to be larger than the
current distance between the transducers 10,20.
Therefore, there is tradeoff in increasing N. Increasing N
decreases distance error due to ultrasound interference, but also
increases distance error due to distance variation within the
measurement window by increasing the size of the measurement
window.
One way to reduce error caused by distance variation within the
measurement window is to shorten the measurement window. This may
be accomplished without decreasing N,. e.g., by increasing the
transmit rate of the system 710 in order to provide more distance
measurements within a shorter period of time. This way, distance
error due to distance variation can be reduced without increasing
distance error due to ultrasound interference.
In another embodiment, the distance filter 715 computes a distance
variation within the measurement window, and compares the computed
distance variation to a threshold value. The distance filter 715
outputs the maximum distance measurement only when the distance
variation within the measurement window exceeds the threshold
value. Otherwise, the filter 715 outputs the most recent distance
measurement from the distance circuitry 340.
The threshold value may be determined by the maximum that the
distance between the transducers 10,20 can change within the
measurement window due to movement between the transducers 10,20.
For example, when the receiving transducer 20 is mounted on a
catheter (not shown), the maximum change in distance may be
determined by the maximum distance that a physician navigates the
catheter within the measurement window. The threshold value may be
represented as threshold=N.delta. where .delta. represents the
maximum change in distance between adjacent distance measurements
and N is the number of distance measurements considered.
The operation of the distance filter 715 according to this
embodiment will now be described with reference to FIG. 8. In step
810, the distance filter 715 computes a distance variation for the
last N distance measurements from the distance circuitry 340. The
distance variation may be computed as the difference between the
maximum and minimum of the last N distance measurements. This is
represented as range(n)=max[x(n),x(n-1), . . .
,x(n-N)]-min[x(n),x(n-1), . . . ,x(n-N)] where range(n) is the
distance variation for the last N distance measurements, and x(n)
is the most recent distance measurement from the distance circuitry
340. Other measures of distance variation may be used, such as
computing the variance of the last N distance measurements. As
another example, the second derivative of the last N distance
measurements, which can be used to differentiate between rapid
catheter movement and random interference can be taken. This is
because rapid catheter movement, in the absence of interference,
produces a distance measurement with a small second derivative,
whereas random interference produces a distance measurement with a
relatively large second derivative.
In step 820, the distance filter 715 compares the distance
variation to the threshold value. If the distance variation is
above the threshold value, the distance filter 715, at step 830,
outputs the maximum of the last N distance measurements from the
distance circuitry 340. Otherwise, at step 840, the distance filter
715 outputs the most recent distance measurement from the distance
circuitry 340. The distance filter 715 repeats the steps in FIG. 8
for each subsequent distance measurement it receives from the
distance circuitry 340.
Therefore, the distance filter 715 according to this embodiment
reduces the unwanted side effects of increasing N by only
outputting the maximum distance measurement when the computed
distance variation exceeds the threshold value. In other words, the
distance filter 715 only applies its maximum filtering function
when the distance variation is most likely due to ultrasound
interference, and not due to movement between the transducers
10,20.
In the discussion above, the systems had one transmitting
transducer. Many applications, such as ultrasound positional
tracking, require multiple transmitting transducers. FIG. 9
illustrates an embodiment of the system 905, further including
multiple transmitting transducers 10-1 to 10-n, where each of the
transmitting transducers is coupled to a pulse generator 920-1 to
920-n. In this embodiment, the control and timing circuitry 950
sequentially initiates distance measurements between the receiving
transducer 20 and each of the transmitting transducers 10-1 to
10-n.
In operation, the control and timing circuitry 950 sequentially
triggers each one of the pulse generators 920-1 to 920-n to
generate and transmit a transmit pulse to its respective
transmitting transducer 10-1 to 10-n causing the transmitting
transducer 10-1 to 10-n to transmit an ultrasound pulse (not
shown). The transmit pulses may be spaced apart in time, e.g., 1
ms, so they do not interfere within one another.
For each transmit pulse, the control and timing circuitry 950
triggers the distance counter 360 to begin counting from zero at
the transmit time of the transmit pulse. The control and timing
circuitry 950 may also send data to the distance circuitry 340
identifying the corresponding transmitting transducer 10-1 to 10-n.
After the transmit pulse has been transmitted, the distance
circuitry 340 listens for a receive pulse within a time window,
e.g., 100 .mu.s. Upon detection of the receive pulse by the
threshold detector 330, the distance circuitry 340 reads the
current count value from the distance counter 360, which provides a
distance measurement between the receiving transducer 20 and the
corresponding transmitting transducer 10-1 to 10-n. The distance
circuitry outputs the distance measurement to the distance filter
915. The distance circuitry 340 may also output data identifying
the transmitting transducer 10-1 to 10-n corresponding to the
distance measurement.
The distance filter 915 sequentially receives distance measurements
corresponding to the distance between the ranging transducer 20 and
each of the transmitting transducers 10-1 to 10-n from the distance
circuitry 340. For each received distance measurement, the filter
915 identities the corresponding transmitting transducer 10-1 to
10-n. This may be accomplished several ways. For example, for each
distance measurement, the control and timing circuitry 950 and/or
the distance circuitry 340 may send data to the filter 915
identifying the corresponding transmitting transducer 10-1 to 10-n.
Alternatively, the filter 915 may determine the identity of the
corresponding transmitting transducer 10-1 to 10-2 according to the
order in which it receives a sequence of distance measurements from
the distance circuitry 340. For example, the filter 915 may assume
that the first distance measurement in the sequence corresponds to
transmitting transducer 10-1, the second distance measurement
corresponds to transmitting transducer 10-2, and so forth.
In one embodiment, the distance filter 915 outputs a maximum
distance for each of the transmitting transducers 10-1 to 10-n. In
determining the maximum distance for each transmitting transducer
10-1 to 10-n, the filter 915 takes the maximum of the last N
distance measurements for that particular transmitting transducer
10-1 to 10-n from the distance circuitry 340.
In another embodiment, the 915 filter also computes a distance
variation for each transmitting transducer 10-1 to 10-n by
computing a variation in the last N distance measurements for that
particular transmitting transducer 10-1 to 10-n. The filter 915 may
then compare the distance variation for each transmitting
transducer 10-1 to 10-n to a threshold value. If the distance
variation for a particular transmitting transducer 10-1 to 10-n is
below the threshold value, then the filter 915 outputs the most
recent distance measurement for that transmitting transducer 10-1
to 10-n. If the distance variation for a particular transmitting
transducer 10-1 to 10-n is above the threshold value, then the
filter 915 outputs the maximum of the last N distance measurements
for that transmitting transducer 10-1 to 10-n.
Preferably, each time the filter 915 outputs a filtered distance
measurement it includes data identifying the corresponding
transmitting transducer 10-1 to 10-n. If implemented in software,
the filter 915 is preferably embodied in several software
modules--one for each transmitting transducer.
The invention is particularly well suited for use in ultrasound
positional tracking systems to track the position of one or more
devices (e.g., catheter). FIG. 10 illustrates an ultrasound
positional tracking system 1005 according to an embodiment of the
invention. The system 1005 includes two or more ranging transducers
1020-1 to 1020-m mounted on the medical device being tracked (not
shown), and four or more reference transducers 1010-1 to 1010-n.
Each of the ranging transducers 1020-1 to 1020-m acts as a
receiving transducer and each of the reference transducers 1010-1
and 1010-n can act both as a receiving and transmitting
transducer.
FIG. 11 illustrates exemplary devices on which the ranging
transducers 1020-1 to 1020-m and the reference transducers 1010-1
to 1010-n can be mounted. Three ranging transducers 1020-1 to
1020-3 are mounted on a distal portion of a catheter 1110 for
performing medical and/or mapping procedures within the body. The
catheter 1110 may be a mapping catheter, an ablation catheter, or
the like. In this example, the ranging transducers 1020-1 to 1020-m
take the form of annular ultrasound transducers. Also illustrated
in FIG. 11 are four reference transducers 1010-1 to 1010-4 mounted
on a distal portion of a reference catheter 1120.
Referring back to FIG. 10, the system 1005 also includes a distance
measurement subsystem 1015-1 to 1015-(n+m) coupled to each of the
ranging transducers 1020-1 to 1020-m and reference transducers
1010-1 to 1010-n, and a pulse generator 1025-1 to 1025-n coupled to
each of the reference transducers 1010-1 to 1010-n. Each distance
subsystem 1015-1 to 1015-(n+m) includes a threshold detector 1030-1
to 1030-(n+m), distance circuitry 1035-1 to 1035-(n+m) and a
distance filter 1040-1 to 1040-(n+m) according to the invention for
filtering out distance errors from the respective distance
circuitry 1035-1 to 1035-(n+m). Notably, if implemented in
software, the number of software modules for the distance filter
1040 will equal the product of the number of ranging transducers
1020 and the number of reference transducers 1010, i.e., m.times.n
filter modules. The system 1005 further includes control and timing
circuitry 1050 coupled to each of the pulse generators 1025-1 to
1025-n and a distance counter 1060 coupled to control and timing
circuitry 1050 and each of the distance measurement subsystems
1015-1 to 1015-(n+m). The system 1005 also includes a triangulation
circuitry 1070 for triangulating the positions of the ranging
transducers 1020-1 to 1020-m and the reference transducers 1010-1
to 1010-n, a display image processor 1075 coupled to the
triangulation circuitry, and a display 1080 coupled to the display
image processor 1075. In the preferred embodiment, the
triangulation circuitry 1070 is implemented in software, but can be
implemented in firmware or hardware as well.
In operation, the control and timing circuitry 1050 sequentially
triggers each one of the pulse generators 1025-1 to 1025-n to
generate and transmit a transmit pulse to its respective reference
transducer 1010-1 and 1010-n causing the reference transducer
1010-1 and 1010-n to transmit an ultrasound pulse. The transmit
pulses may be spaced apart in time, e.g., 1 ms, so they do not
interfere within one another.
For each transmit pulse, the control and timing circuitry 1050
triggers the distance counter 1060 to begin counting from zero at
the transmit time of the transmit pulse. After the transmit pulse
has been transmitted, each distance measurement subsystem 1015-1 to
1015-n listens for a receive pulse at its respective transducer
1020-1 to 1020-m and 1010-1 to 1010-n. Upon detection of a receive
pulse by its respective threshold detector 1030-1 to 1030-(n+m),
each distance circuitry 1035-1 to 1035-(n+m) reads the current
count value from the distance counter, and outputs the read count
value as a distance measurement to the respective distance filter
1040-1 to 1040-(n+m). Each distance filter 1040-1 to 1040-(n+m)
filters out erroneous distance measurements from its respective
distance circuitry 1035-1 to 1035-(n+m) due to ultrasound
interference, and outputs the filtered distance measurements to the
triangulation circuitry 1070. Each distance filter 1040-1 to
1040-(n+m), preferably, includes data identifying the corresponding
receiving and transmitting transducer for each filtered distance
measurement.
For each transmit pulse, the triangulation circuitry 1070 receives
distance measurements between the reference transducer 1010-1 to
1010-n corresponding to the transmit pulse and each of the other
reference transducers 1010-1 to 1010-n. The triangulation circuitry
1070 also receives distance measurements between the reference
transducer 1010-1 to 1010-n corresponding to the transmit pulse and
each of the ranging transducers 1020-1 to 1020-m.
The triangulation circuitry 1070 computes the relative positions
between the reference transducers 1010-1 to 1010-n in 3-D space by
triangulating the distance measurements between the references
transducers 1010-1 to 1010-n. The triangulation circuitry 1070 then
maps the relative positions between the reference transducers
1010-1 to 1010-n onto the 3-D coordinate system to provide a
reference for tracking the positions of the ranging transducers
1020-1 to 1020-m in the 3-D coordinate system. The triangulation
circuitry 1070 may employ any one of a number of mapping procedures
as long as the mapping procedure preserves the relative positions
between the reference transducers 1010-1 to 1010-n.
To track the positions of the ranging transducers 1020-1 to 1020-m
within the 3-D coordinate system, the triangulation circuitry 1070
triangulates the relative positions of the ranging transducers
1020-1 to 1020-m to the reference transducers 1010-1 to 1010-n by
triangulating the distance measurements between the ranging
traducers 1020-1 to 1020-m and the reference traducers 1010-1 to
1010-n. The triangulation circuitry 1070 then determines the
positions of the ranging transducers 1020-1 to 1020-m in the 3-D
coordinate system based on the relative positions of the ranging
transducers 1020-1 to 1020-m to the reference transducers 1010-1 to
1010-n and the known positions of reference transducers 1010-1 to
1010-n in the 3-D coordinate system.
Those skilled in the art will appreciate that the filters 1040-1 to
1040-(n+m) may be integrated in the triangulation circuitry 1070.
This may be done, e.g., by modifying software in the triangulation
circuitry 1070 to perform the filtering functions according to the
invention. In this case, the distance measurements from each
distance circuitry 1040-1 to 1040-(n+m) may be outputted directly
to the triangulation circuitry 1070, which performs the filtering
functions according to the invention before triangulating the
positions of the transducers.
The triangulation circuitry 1070 outputs the positions of the
ranging transducers 1020-1 to 1020-m and the reference transducers
1010-1 to 1010-n in the 3-D coordinate system to the display image
processor 1075. The display image processor 1075 generates an image
showing the position and orientation of the device being tracked in
graphical form. The display image processor 1075 may do this by
plotting the positions of the ranging transducers 1020-1 to 1020-m
in the 3-D coordinate system and reconstructing a graphical
representation of the device onto the plotted positions based on a
pre-programmed graphical model of the device. The graphical model
may include information on the relative positions of the ranging
transducers on the device. The image may also show the position and
orientation of the reference catheter 1120 (shown in FIG. 11) in
graphical form. The display image processor 1075 may do this by
plotting the positions of the reference transducers 1010-1 to
1010-n in the 3-D coordinate system and reconstructing a graphical
representation of the reference catheter 1120 onto the plotted
positions. The display image processor 1075 outputs the image to
the display 1080, which displays the image to a physician.
FIG. 12 shows an exemplary image 1210 in which the device being
tracked is a medical catheter 1110 (shown in FIG. 11). The image
1210 includes graphical reconstructions of the medical catheter
1110 and the reference catheter 1120. The graphical reconstruction
of the medical catheter 1110 is positioned and orientated in the
image 1210 based on the tracked positions of the ranging
transducers 1020-1 to 1020-3 in the 3-D coordinate system.
Similarly, the graphical reconstruction of the reference catheter
1120 is positioned and orientated in the image 1210 based on the
tracked positions of the reference transducers 1010-1 to 1010-4 in
the 3-D coordinate system. The 3-D coordinate 1215 system may or
may not be shown in the image 1210.
Even though one device was tracked in the above example, the
ultrasound positional system 1005 may be used to track multiple
devices equipped with ranging transducers. In addition, the display
1080 may display the position of anatomical landmarks in the 3-D
coordinate system. This may be done, e.g., by positioning a mapping
catheter equipped with ranging transducers at an anatomical
landmark and recording the position of the anatomical landmark in
the 3-D coordinate system based on the position of the mapping
catheter. The position of the anatomical landmark in the. 3-D
coordinate system may then be displayed and labeled on the display
1080. This enables a physician to more precisely guide devices
within the body by referencing their tracked position on the
display 1080 to the position of the anatomical landmark on the
display 1080.
The display 1080 may also display a computer representation of body
tissue in the 3-D coordinate system. This may be done, e.g., by
moving a mapping catheter equipped with ranging transducers to
different positions on the surface of the body tissue and recording
these positions in the 3-D coordinate system. The image display
processor 1075 may then reconstruct the computer representation of
the body tissue in the 3-D coordinate system, e.g., by fitting an
anatomical shell onto the recorded positions. The computer
representation of the body tissue may then be displayed on the
display 1080. This enables a physician to more precisely guide
devices within the body by referencing their tracked position on
the display 1080 to the computer representation of the body tissue
on the display 1080. Additional details on this graphical
reconstruction technique can be found in patent application Ser.
No. 09/128,304 to Willis et al. entitled "A dynamically alterable
three-dimensional graphical model of a body region", which is
incorporated by reference.
An advantage of the ultrasound tracking system 1005 according to
the invention is that the distance filters 1040-1 to 1040-(n+m)
enable the tracking system 1005 to more reliably operate in an
environment containing ultrasound interference. This is
accomplished by filtering out distance errors due to ultrasound
interference, thereby improving the accuracy of the distance
measurements used to triangulate the positions of the ranging
transducers 1020-1 to 1020-n and the reference transducers 1010-1
to 1010-n.
The invention is especially useful for using ultrasound tracking
systems concurrently with ultrasound imaging. One advantage of
using an ultrasound tracking system concurrently with ultrasound
imaging is that it allows a physician to track the position of a
device within a portion of the body while at the same time imaging
the portion of the body using a ultrasound imager to provide
additional information. Another advantage is that it allows the use
of an ultrasound tracking system to track the position of a device
having an ultrasound imager, e.g., an ultrasound imaging
catheter.
The usefulness of the invention in using an ultrasound ranging
system concurrently with ultrasound imaging will now be
examined.
An ultrasound imager typically images the body by transmitting
ultrasound pulses in the body and detecting the resulting echo
pulses. The rate of transmission of the ultrasound pulses is
constrained by two factors.
1. The distance that is to be imaged. This is because there must be
enough time for the ultrasound energy to travel out to and back
from the object being imaged.
2. Scattering interference. This is because the scattered energy
from one ultrasound pulse must die out before the next ultrasound
pulse can be transmitted.
The more rapidly the imaging transducer is pulsing, the higher the
probability it will cause interference at an ultrasound tracking
system. The probability of ultrasound interference occurring
between two transducers of an ultrasound ranging system is
P=(d/v)/T where d is distance between the transducers, v is the
velocity of ultrasound between the transducers, and T is the time
between ultrasound pulses from the interfering ultrasound
imager.
In one example, an Intracardiac Echocardiography ("ICE") catheter
contains an ultrasound imager that transmits at a rate of 7680 Hz,
which corresponds to a time, T, of 130 .mu. is between ultrasound
pulses from the imager. FIG. 13 is a graph showing the probability
of this particular ultrasound imager causing interference between
two transducers of an ultrasound ranging system as a function of
distance between the transducers. Without the distance filter of
the invention, the probability of a distance measurement error is
P=(d/v)/T, as given by the above equation. This is shown for v=1.5
mm/.mu.sec and T=130 As by the curve labeled "unfiltered" in FIG.
13. The value v=1.5 mm/.mu.sec is an approximation of the velocity
of ultrasound in the body. With the distance filter of the
invention, the probability of a distance measurement error is
significantly reduced to P to the Nth power. This is shown for N=8
by the curve labeled "filtered" in FIG. 13.
Those skilled in the art will appreciate that various modifications
may be made to the just described preferred embodiments without
departing from the spirit and scope of the invention. For example,
the distance filters of the invention are not limited for use with
the particular ultrasound ranging systems described in the
specification, and may be used with other ultrasound ranging
systems susceptible to ultrasound interference. Therefore, the
invention is not to be restricted or limited except in accordance
with the following claims and their legal equivalents.
* * * * *